# General Relativity

**Topics:**General relativity, Gravitation, Black hole

**Pages:**11 (3674 words)

**Published:**February 17, 2013

Pedro Ferreira

INSTANT EXPERT 1

EinstEin’s insight

In 1905, at the age of 26, Albert Einstein proposed his special theory of relativity. The theory reconciled the physics of moving bodies developed by Galileo Galilei and Newton with the laws of electromagnetic radiation. It posits that the speed of light is always the same, irrespective of the motion of the person who measures it. Special relativity implies that space and time are intertwined to a degree never previously imagined. Starting in 1907, Einstein began trying to broaden special relativity to include gravity. His first breakthrough came when he was working in a patent office in Bern, Switzerland. “Suddenly a thought struck me,” he recalled. “If a man falls freely, he would not feel his weight… This simple thought experiment… led me to the theory of gravity.” He realised that there is a deep relationship between systems affected by gravity and ones that are accelerating. The next big step forward came when Einstein was introduced to the mathematics of geometry developed by the 19th-century German mathematicians Carl Friedrich Gauss and Bernhard Riemann. Einstein applied their work to write down the equations that relate the geometry of space-time to the amount of energy that it contains. Now known as the Einstein field equations, and published in 1916, they supplanted Newton’s law of universal gravitation and are still used today, nearly a century later. Using general relativity, Einstein made a series of predictions. He showed, for example, how his theory would lead to the observed drift in Mercury’s orbit. He also predicted that a massive object, such as the sun, should distort the path taken by light passing close to it: in effect, the geometry of space should act as a lens and focus the light (see diagram). Einstein also argued that the wavelength of light emitted close to a massive body should be stretched, or red-shifted, as it climbs out of the warped space-time near the massive object. These three predictions are now called the three classical tests of general relativity.

”space tells matter how to move and matter tells space how to curve” John Archibald Wheeler

Einstein suggested that light rays skimming past the sun would be bent by its gravity. To test the idea, Arthur Eddington rst photographed the Hyades stars at night. He then needed to photograph them when they were on the far side of the sun. For this picture, it only became possible to see the starlight when the glare of the sun was eliminated by a total solar eclipse. His images con rmed Einstein’s prediction According to general relativity, space-time can be viewed as a smooth, exible sheet that bends under the in uence of massive objects

APPARENT POSITION

HYADES STAR CLUSTER, 150 LIGHT YEARS AWAY ACTUAL POSITIONS APPARENT POSITION

SUN

EARTH

The mass of the sun bends space-time, so bright rays from the Hyades cluster bend too. Viewed from Earth, the stars appear to have shifted

ii | NewScientist

HISTORY OF GENERAL RELATIVITY

Albert Einstein’s general theory of relativity is one of the towering achievements of 20th-century physics. Published in 1916, it explains that what we perceive as the force of gravity in fact arises from the curvature of space and time. Einstein proposed that objects such as the sun and the Earth change this geometry. In the presence of matter and energy it can evolve, stretch and warp, forming ridges, mountains and valleys that cause bodies moving through it to zigzag and curve. So although Earth appears to be pulled towards the sun by gravity, there is no such force. It is simply the geometry of space-time around the sun telling Earth how to move. The general theory of relativity has far-reaching consequences. It not only explains the motion of the planets; it can also describe the history and expansion of the universe, the physics of black holes and the bending of light from distant stars and galaxies.

Gravity...

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